This invention relates to a muffler for use in conjunction with a high intensity, low frequency noise-propagating open pipeline. In particular, it relates to a muffler, for use in conjunction with an exhaust pipe leading from one or more vacuum pumps. For example, a vacuum system is used to convey polymer chips from one area to another in a plant. The muffler reduces by up to 30 dB and 11.DELTA. dB(A) the noise emitted at the exit of the exhaust pipe.
Throughout the present specification and claims, the term "dB" (decibels) connotes the unit of measurement of sound pressure level as defined in ANSI Sl.4-1971, using a reference level of 20 micropascals (2.times.10.sup.-5 Newtons per square meter). The term "dB(A)" (decibels-A-weighted) connotes a unit of measurement of sound level corrected to the A-weighted scale, as defined in ANSI Sl.4-1971, using a reference level of 20 micropascals (2.times.10.sup.-5 Newtons per square meter). The term ".DELTA. dB(A)" refers to the difference between two noise levels where each level is expressed in units of dB(A).
The muffler of this invention is useful any time an open pipe propagates noise, such as exhaust pipes from vacuum pumps which are widely used in industry. To convey polymer chips, the inlet of each vacuum pump is connected to a conveyance system and a partial vacuum is put on the system when the vacuum pump is started. The partial vacuum creates an unbalanced force to suck or drag polymer chips through the system. The pump compresses air to impart force to the air and consequently generates pulses, or sound waves. It is this noise that propagates down the exhaust pipe of the vacuum pump. The noise emitted, as measured approximately 15.24 centimeters (6 inches) to one side of the exhaust pipe exit, was 115 to 116 dB for an octave band center frequency of 125 cycles per second (hereafter cps), and 113 dB for an octave band center frequency of 250 cps, and 99 dB(A). As can be seen, the major noise component is of high intensity and in the low frequency range. Low frequency noise, while less harmful than high frequency noise, can be heard and is very annoying, especially in an otherwise quiet atmosphere.
The vacuum pump may be an intermittent or continuous source of noise. In either event, as a major contributor of noise or as a minor component in an area which has several other sources of noise, it is desirable to bring the noise level down to more acceptable limits.
Report 1192 of the National Advisory Committee for Aeronautics discloses mufflers which combine a resonator with an expansion chamber. U.S. Pat. No. 4,116,303 to Trudell discloses an exhaust muffler which provides two expansion chambers in series, wherein the second chamber has the dual function of serving as the second expansion chamber to improve low frequency noise reduction and as a resistive absorber to improve middle and high frequency noise reduction.